The aim of this research is to investigate the contractile activity of cardiac muscle at the single cell level and at the sarcomere level within the single cell. Isolated cardiac cells from adult frog atrial tissue will be prepared by dispersion of intact tissue with trypsin and collagenase. The specific areas of investigation will be: 1) contractile activity elicited by electrical stimulation, 2) contractures, and 3) contractile activity as related to the kinetics of sarcomere shortening and relaxation during a contraction cycle. The objective of this research is to use the single cardiac cell as a means of 1) gaining additional insight into the factors (e.g., ionic environment, drugs, etc.) which control contractile activity of cardiac muscle, and 2) gaining additional insight into the factors relating mechanical performance at the sarcomere level to mechanical performance at the cellular level.